This invention relates to cemented carbide materials, and in particular to bodies fabricated of metal-cemented carbide materials in which the fracture toughness of the body surface has been increased by enrichment of the metal binder component in that region. The invention also relates to a method for manufacturing such surface-toughened bodies.
In the cemented carbide tool industry, high toughness is generally achieved with straight WC-Co grades, which are fully dense composites of tungsten carbide grains and a metal, typically cobalt, binder. Improved chemical wear resistance and high deformation resistance are addressed with multi-carbide steel cutting grades, for example WC-Co composites containing at least 10 w/o (weight percent) .beta.-phase. The so-called .beta.-phase materials are carbides having a "rock-salt" crystal structure, and are generally called B-1 carbides in the cutting tool industry. These are the carbides of titanium, zirconium, hafnium, vanadium, niobium, and tantalum. The most common B-1 carbides used in the cutting tool industry are TiC, TaC, and NbC.
The application of hard refractory coatings, for example TiC or dual layer coatings of TiC/Al.sub.2 O.sub.3, to cutting tools, generally by chemical vapor deposition (CVD), has been used to improve the wear resistance of the tools. The application of hard refractory coatings to cemented carbide cutting tool substrates greatly reduces the effect of many of the wear processes, for example chemical/diffusion wear, which are of concern when dealing with uncoated cutting tool grades. This frees the tool manufacturer to tailor the substrate microstructure to achieve both high toughness and high deformation resistance.
The application of a refractory coating, however, can itself significantly reduce the toughness of a carbide tool, for example reducing the chipping or breakage resistance of the tool by as much as 20-50%. Accordingly, considerable effort has been directed to development of substrates with even further increased toughness to offset the toughness decreasing effects of the coating process. Such high toughness along with high deformation resistance may be achieved by surface toughening of a substrate having a deformation-resistant core.
In one type of surface toughening process a B-1 carbide containing substrate, for example a WC-Co substrate containing about 10 w/o total TiC and TaC, is treated to cause removal of the B-1 carbides from the substrate surface by migration of these carbides toward the core of the tool. During this treatment, binder, in turn, migrates toward the surface. Thus a near-surface layer is produced, typically 20-50 microns in depth, having a microstructure devoid of B-1 carbides and enriched in binder content (about twice that of the bulk). This layer devoid of B-1 carbides is called a .beta.-free layer (.beta.FL). The binder enrichment in this layer results in a tool exhibiting high toughness.
Another type of surface toughening process for B-1 carbide containing substrates is effected in the presence of so-called "C-porosity". The term "C-porosity" refers to free carbon present in the microstructure. This free carbon is excess carbon, that is an amount beyond the solubility limit of carbon in the binder, precipitated from the liquid phase during cooling from the high sintering temperature. Such C-porosity is described in further detail in ASTM B 276-86, incorporated herein by reference. This C-porosity is known to be present in tungsten carbide-cobalt substrates containing about 10 w/o B-1 carbides, and has been shown to produce heavy binder enrichment (about three times that of the bulk) in the surface layers of such substrates during sintering. The presence of B-1 carbides has thus been considered necessary for such binder enrichment by those skilled in the art.
The microstructure of these surface binder-enriched substrates exhibits a binder content which decreases gradually with the depth from the surface until it reaches the bulk value. In the region of increased binder content, the article exhibits a stratified microstructure with the metal binder appearing as "wavelets" in the binder-enriched zone. The enriched zone contains some B-1 carbides, but their concentration decreases gradually from the bulk value to essentially zero at the surface.
The increase in binder content in the surface layer increases the resistance to fracture of the outer substrate layer, (a) inhibiting propagation into the substrate of cracks inherent in brittle refractory coatings applied to the substrate surface, and (b) increasing the impact resistance of the coated tool. Since the toughened surface layer below the coating is thin, the properties inherent in the microstructure of the bulk of the substrate predominate, and the required deformation resistance is maintained.
As mentioned above, it has been generally accepted by those skilled in the art that such binder-enriched surface layers may be achieved only in the presence of B-1 carbides, whether by creation of a .beta.-free layer or in the presence of C-porosity.
U.S. Pat. No. 4,277,283 (Tobioka et al.) describes .beta.FL layers produced by adding 4-6.3 w/o solid solution carbonitride, (Ti.sub..75 W.sub..25)(C.sub..68 N.sub..32), to a mixture of (Ta.75Nb.25)C, cobalt, and WC. This produced a .beta.FL surface layer devoid of B-1 transition metal carbonitride phase. Other compositions containing only WC and solid solution carbonitride with cobalt produced a .beta.FL layer, but these all contained at least 10 w/o B-1 carbonitride.
U.S. Pat. No. 4,558,786 (Yohe) describes surface toughening of cobalt bonded tungsten titanium carbide substrates containing TaC and (W,Ti)C by B-1 phase depletion and binder enrichment.
U.S Pat. No. 4,497,874 (Hale) also describes binder enrichment surface toughening in a composition of TiC (or (W,Ti)C), TaC, cobalt, and WC.
U.S. Pat. No. 4,610,931 (Nemeth et al.) describes binder-enriched surfaces in cemented carbides containing Co, a chemical agent, B-1 carbides or solid solution carbides, and WC. The chemical agent is a transition metal or solid solution, or their hydride, nitride, or carbonitride which is at least partially converted to the metal carbide on sintering. Free carbon may be added to convert added metals, hydrides, nitrides, or carbonitrides to B-1 carbides.
U.S. Pat. No. 4,150,195 (Tobioka et al.) describes adding excess carbon to cemented carbide substrates to increase toughness. No binder enrichment is described.
Nemeth et al. (10th Plansee Seminar Proc., 1, p. 613, 1981) describe a B-1 containing cemented carbide cutting tool having a substrate partially surface-toughened through binder enrichment.
Grab et al. (High Productivity Machining, ed. V. K. Sarin, ASM, p. 113, 1985) discuss binder-enriched, surface-toughened substrates of a composition similar to that described by Nemeth et al., referenced immediately above.
Suzuki (Trans. Japan Inst. of Metals, 22 (11) pp. 758-764, 1981) describe cemented carbides exhibiting a .beta.FL layer and including B-1 solid solution carbonitrides. Similar materials are reported by Tsukado et al. (Sumitomo Electric Tech. Rev. #24, Jan. 1985).
All of these references describe cemented carbides which are surface toughened by binder enrichment and .beta.FL formation, which is the creation of a surface layer devoid of B-1 carbide phase. The described cemented carbides all contain Co, WC, and appreciable amounts of B-1 carbides. The amounts of carbides, etc. are expressed in weight percent in these references. Since the density of TiC is about 5 g/cm.sup.3, that of TaC is about 15 g/cm.sup.3, and that of WC is about 15 g/cm.sup.3, the TiC-containing formulations in these references are particularly high in volume percent of B-1 carbides. This limits the opportunity for achieving the advantages of surface toughening to only those compositions containing sufficient B-1 phase such that B-1 phase migration may be effected and a .beta.FL developed. It would be advantageous to develop other cemented carbide compositions, for example B-1 carbide free compositions, in which surface binder-enrichment may be produced.